I've been through this with 5 elderly dogs in the last year. Each time, we've ditched the low-protein options in favour of "whatever she'll eat." There are a few medical options that can help: sub-q fluids, Mirtazapine (a human anti-depressant that can stimulate appetite in dogs; I haven't had much luck with it but continue with it anyway), and anabolic steroids (stimulate appetite and improve strength). The steroids kept Jessie going for a long time, but we caught her kidney disease early and when she was a bit younger than some of the others.